This invention relates to a heat-resistant, self-extinguishing resin coated electric wire.
Since polyethylene, butyl rubber and ethylene-propylene rubber have excellent insulating properties, they are widely used as insulating material for electric wires. However, since they are combustible, they are likely to be flame-spread. Therefore, when they give rise to dielectric breakdown, this tends to become the origin of a fire. In order to overcome this disadvantage in the prior art, an attempt has been made to impart self-extinguishing property to a resin coated on electric wire. In general, such self-extinguishing property has been imparted to the resin by adding a self-extinguishing agent or a flame retardant to the resins.
However, when the self-extinguishing or flame retardant is added to the resin, the insulating property of the resin is lowered. For example, in order to impart self-extinguishing property to the resin without impairing the insulating property of the resin coated on the wire, a process which comprises coating a self-extinguishing polyvinyl chloride resin on electric wire having an insulating layer has been proposed. The reason why the electric wire having a self-extinguishing property polyvinyl chloride and a combustible, insulating resin thereon is burned only modestly is considered to be the following: When flames are struck against the wire, the polyvinyl chloride and the combustible, insulating resin are melted and heat-decomposed. The combustible resin generates a combustible gas through the heat-decomposition. When the gas passes through the outer non-flammable polyvinyl chloride, it reacts with a flame-retardant or a self-extinguishing agent present in the non-flammable polyvinyl chloride to form a self-extinguishing gas or the gas is mixed with a non-flammable gas or a self-extinguishing gas generated from the polyvinyl chloride, whereby combustibility of the gas generated from the combustible resin is lost. An electric cable as a whole can be made self-extinguishing by coating the self-extinguishing polyvinyl chloride as an outer layer on the electric wire.
In general, when polyvinyl chloride is used as a coating material for electric wire, the workability and flexibility of the chloride is necessary to be increased by adding a plasticizer to the resin. Such plasticizer includes plasticizers of phthalic acid type, such as dioctyl phthalate, of trimellitic acid type and of polyester type.
A copolymer of vinyl chloride and vinyl acetate, ethylene, propylene or an acrylic monomer can be used as a flexible polyvinyl chloride in place of adding a plasticizer to polyvinyl chloride.
Also, a flexible resin can be obtained by grafting vinyl chloride to an ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer or ethylene-propylene copolymer. Such a vinyl chloride-grafted copolymer may be used as a self-extinguishing resin.
Though polyvinyl chloride is little burnt itself, self-extinguishing property of the resin is lowered by adding a plasticizer thereto. Some of the plasticized resin may be combustible. In order to increase the self-extinguishing property of polyvinyl chloride, an inorganic flame retardant, such as antimony trioxide or an organic flame retardant, such as an organic halogen compound is generally added to the plasticized polyvinyl chloride.
The term "a polyvinyl chloride series resin" or "vinyl chloride series polymer" means a polymer containing vinyl chloride units, such as a vinyl chloride-grafted polymer or a copolymer of vinyl chloride and an other monomer. In general, since the softening temperature of the polyvinyl chloride series resin is low, the resin is likely to be melted by heating the resin at a slightly elevated temperature. It has been known in the prior art that the polyvinyl chloride series resin is crosslinked in order to overcome the above disadvantages.
As mentioned above, though polyethylene, butyl rubber and ethylene-propylene rubber have excellent insulating properties, they are combustible. However, a mixture of the above polymer or chlorinated polyethylene, etc. with an inorganic flame retardant, such as antimony trioxide or an organic flame retardant, such as an organic halogenated compound, is self-extinguishing. These polymers have low softening temperatures. The softening temperature of the polymer is increased by introducing the crosslinking linkage thereinto.
We have found that when the degree of crosslinking of a polyvinyl chloride series resin is made high in order to promote the resistance to heat distortion of the resin, the self-extinguishing property of the resin is lowered, that is, the resin becomes likely to be flame-spread. We have carried out research on the relationship between the degree of crosslinking said resin and the time that combustion of the crosslinked resin continues. The results are shown in FIG. 1. The shorter the time that combustion of the crosslinked resin continues, the greater the nonflammability. The higher the gel percent of the resin, the greater the degree of cross-linking the resin.
We have also carried out research on the relationship between degree of cross-linking polyethylene and time of maintaining combustion of the crosslinked polyethylene. The results are shown in FIG. 3.
These FIGS. 1 and 3, show that the greater the degree of crosslinking the resin, the poorer the self-extinguishing property of the polymer. These FIGS. 1 and 3 also show that the nonflammability of the resin is enhanced by crosslinking the resin to a suitable extent. The degree of crosslinking the resin sufficient to obtain excellent resistance to heat distortion corresponds to gel percent of more than 70%. However, FIGS. 1 and 3 show that a resin having a gel percent of 70% is inferior to the non-crosslinked resin in respect of self-extinguishing property.
The reasons theorizing the relationship between the a degree of crosslinking a resin and the time that combustion of the crosslinked polymer continues are unclear at present. When the combustion gas generated from the combustible resin passes through the layer of the polyvinyl chloride, it is thought that penetration of the combustion gas into the polyvinyl chloride is suppressed according to the degree of crosslinking of the chloride. That is, penetrating of the combustion gas into the highly crosslinked polyvinyl chloride is prevented.
On the other hand, flames are struck against an electric wire on which an combustible, insulating resin and a polymer of vinyl chloride type are coated in the order of description, the two resins melt and decompose. However, when the polyvinyl chloride series resin is highly crosslinked, flowing of the resin can be avoided even when the resin melts.
Excessive crosslinking of the polyvinyl chloride series resin lowers the flexibility of the resin, whereby internal pressure derived from expansion of the combustible gas generated from the combustible resin is likely to give rise to cracking of the crosslinked resin. Therefore, the combustible gas which passes through a crack in the crosslinked polymer causes the combustion of the polymer-coated electric wire.
On the other hand, when flames are applied to the non-crosslinked resin coated on an electric cable, the non-crosslinked resin easily melts and flows, whereby the thickness of the nonflammable resin layer becomes non uniform. Particularly, the portion of the self-extinguishing resin against which flames are struck becomes thin in thickness. Therefore, the self-extinguishing resin layer having nonuniform thickness can not prevent the combustible gas generated from the combustion resin from leaking out. As a result, the non-crosslinked resin is substantially inferior to the resin crosslinked in respect of the self-extinguishing property.
On the other hand, since the resin crosslinked to a suitable extent has flexibility, the crack in the resin is not formed even in case of striking a flame against the resin.